Technological Field
The present disclosure relates to a method for detecting at least one of a heart rate and a respiratory rate of a subject.
Description of the Related Technology
In recent years, contactless vital signs monitoring has been an increasingly active field of research. The sensing of vital signs can be made contactless and therefore non-invasive by adopting radar techniques. The Doppler shifts caused by the mechanical movements of the heart and the lungs can be detected and analyzed to determine the heart rate and the respiration rate.
A continuous-wave (CW) radar, also known as a Doppler radar, transmits a radio frequency single-tone continuous-wave signal which is reflected by a target and then demodulated in a receiver. By the Doppler effect, the radio frequency signal reflected by the moving tissue of the target undergoes a frequency shift proportional to the surface velocity of the tissue. If the moving tissue has a periodic motion (as the tissue in the chest region of a subject may have due to the periodic motion of the heart and the lungs) the Doppler effect results in a phase shift of the reflected radio frequency signal which is proportional to the instantaneous surface displacement. In the receiver, the transmitted signal may be mixed with the reflected Doppler-shifted signal to produce a mixing product which, following low pass filtering, results in a baseband signal including a low frequency component that is directly proportional to the instantaneous surface displacement.
However, extraction of the low frequency component from the baseband signal in the Doppler radar-based approach requires that the maximum amplitudes of the chest region displacements due to the heart beat and the respiration are much smaller than the wavelength of the radio frequency signal. This may be referred to as the small angle approximation. Assuming, for a typical subject, an average maximum amplitude of the chest tissue displacements due to the heart beat and the respiration of about 0.08 mm and 0.8 mm, respectively, this condition may be easily satisfied by, for example, using a radio frequency signal with λ=0.125 in (2.4 GHz) yielding a maximum phase shift of approximately 5 degrees. The baseband signal may still include some non-linear terms (such as inter-modulation products between the heart rate and the respiration rate), but the terms which are linearly proportional to the instantaneous tissue displacement due to the heart rate and the respiration rate will tend to dominate. However, a tissue displacement of merely 8 mm will produce a phase shift of about 46 degrees and violate the small angle approximation. This implies that in case of random movements of the subject causing a random displacement of the reflecting tissue, reliable extraction of the heartbeat and respiration rates from the baseband signal is severely hampered.
A further condition for extraction of the low frequency component from the baseband signal in the Doppler radar-based approach is that the fixed phase offset between the transmitted signal and the reflected signal (i.e. the part of the phase shift not being due to the Doppler-shift, such as the mean distance between the radar and the subject, the reflection at the subject and radio block delay) is an odd multiple of π/2. This may be referred to as the optimum operation point (or shorter “optimum point”) of the Doppler radar. Unless this condition is met, a mathematical analysis of the mixing product reveals that the baseband signal will be distorted by non-linear terms doubling and mixing the frequency components corresponding to the heart rate and the respiration rate. Furthermore frequency components corresponding to the heart rate and the respiration rate will be multiplied by the total residual phase noise between the transmitter and the receiver, thereby degrading the signal-to-noise-ratio. This issue will be particularly pronounced when the fixed phase offset between the transmitted signal and the reflected signal is an integer multiple of π. This may be referred to as the null operation point (or shorter “null point”) of the Doppler radar.
The null points and the optimum points are distributed alternately and are separated by λ/8, where λ represents the wavelength of the transmitted signal. At the commonly used operating radio frequencies, the distance between an adjacent null point and optimum point is in the order of few millimeters or centimeters. For example, at 2.4 GHz this distance is about 1.5 cm. Therefore, obtaining a reliable measurement at the optimum point is in practice very difficult to achieve. Meanwhile, reducing the operating frequency will increase null point-optimum point separation but also will decrease the sensitivity in detecting the vital signs parameters.
Wu et al. proposes in “Phase- and Self-Injection-Locked Radar for Detecting Vital Signs with Efficient Elimination of DC Offsets and Null Points” (IEEE Transactions on Microwave Theory and Techniques, Vol. 61, No. 1, pp. 685-695, January 2013) an alternative Doppler radar system for vital signs monitoring which employs a phase- and self-injection-locked (PSIL) oscillator. A fine tuning voltage for a dual-tuning voltage-controlled oscillator (VCO) is controlled by a phase-locked loop (PLL) to extract the Doppler-shifted signal. The output signal of the VCO is fed to both the transmitting antenna and a phase frequency detector (PFD) of the PLL. The received Doppler-shifted signal is injected into the VCO through a circulator to form an SIL loop. The SIL loop is phase-locked by the PLL to stabilize the output frequency. A Doppler-shifted injection signal will result in an output phase perturbation of the VCO. The phase perturbation is detected by the PFD comparing the Doppler-shifted injection signal to an output signal of a fixed frequency reference oscillator. A charge pump (CP) circuit and a loop filter transforms the output of the PFD into a fine tuning voltage for tuning the intrinsic oscillation frequency of the VCO. Provided the maximum amplitude of the displacement of the target is much smaller than the free-space wavelength of the transmitted signal, the VCO fine tuning voltage controlled by the PLL reflects the phase variation of the Doppler signal due to the heartbeat. Hence, this architecture also relies on the small angle approximation. Furthermore, the PSIL radar exhibits “null points” since there will be points at which there is a zero power spectral SNR gain wherein detection of a displacement is prevented. Therefore, a path diversity switch is employed to periodically switch between two transmission paths presenting a phase difference of π/2. However, the null point problem may still only be mitigated by the path diversity switch provided the small angle approximation is valid.